Visits to the outer solar system are usually conducted as NASA flagship missions that cost billions of dollars, like the recently concluded Cassini mission to Saturn or the Europa Clipper spacecraft set for launch in the 2020s.

While these projects have produced significant achievements, smaller, less pricey missions also might advance planetary science. On Mars, for instance, no single spacecraft did everything, but in aggregate and over time, the robots sent there revealed the planet’s watery past and set the stage for future astronauts.

That’s why the scientists behind Trident proposal, which will be formally presented to NASA later this month, are seeking support under the agency’s highly competitive Discovery program, for missions that are supposed to cost less than $500 million.

NASA is changing one of the key scientific instruments on Europa Clipper, its next major mission to the outer planets of the solar system, and has brought in a scientific luminary to lead it, project leaders announced today. Clipper is set to orbit Jupiter and study Europa, the icy Jovian moon, across multiple flybys. Earlier this month, NASA headquarters terminated the mission’s ICEMAG magnetometer instrument, citing overruns in its estimated budget. The move left the spacecraft without an essential tool to study Europa’s interior ocean, where astrobiologists hope extraterrestrial organisms might be found.

Margaret Kivelson, a professor emerita at the University of California, Los Angeles, will lead the effort to develop a simplified magnetometer to replace ICEMAG. The instrument will measure Europa’s magnetic field and gather data on the ocean’s depth and salinity. Kivelson previously led the magnetometer team on the spacecraft Galileo, which orbited Jupiter in the 1990s. She is credited with discovering the ocean beneath Europa’s ice shell.

MER came in the aftermath of failed mission proposals by Ray Arvidson, a professor at Washington University in St. Louis; Larry Soderblom of the U.S. Geological Survey; and Steve Squyres, a professor at Cornell University. Each of the three had been beaten by David Paige of University of California, Los Angeles, whose ill-fated Mars Polar Lander was selected for flight by NASA.

“During an [American Geophysical Union] meeting, I stopped Steve in the hall,” says Arvidson. “I said, ‘I’m a pretty sore loser. How about you?’ And that was the start.” Arvidson, Squyres and Soderblom merged their various teams and set about writing a joint proposal to get a rover on the Martian surface.

Once it reaches the bottom of the pit, Kerber says, Axel would explore the cavern floor, providing humanity’s first close look at the subterranean realms of the moon. The rover would carry six times as much tether as it needs, so however far the bottom of the cavern is, Axel should be able to descend deeply enough to discover what waits below.

“The bottom of the pit is total exploration. We have enough time to just see what the heck is down there. We are thinking a monolith,” Kerber jokes, “or a big door covered in hieroglyphics.”